General constitution of a conventional polymer electrolyte fuel cell stack is explained.
A fuel cell using a polyelectrolyte simultaneously generates electric power and heat by electrochemically reacting a fuel gas containing hydrogen and an oxidant gas containing oxygen, such as air. FIG. 5 is a schematic cross-sectional view for explaining the structure of a unit cell of a solid polymer electrolyte fuel cell. As shown in FIG. 5, a catalyst layer 52 is formed on both sides of a polyelectrolyte membrane 51 selectively transporting hydrogen ion, the catalyst layer 52 comprising a mixture of a catalyst body obtained by having a platinum series metal catalyst carried on a carbon powder, and a hydrogen ion-conductive polyelectrolyte.
At present, polyelectrolyte membranes comprising a perfluorocarbonsulfonic acid (for example, Nafion membrane, a product of Du Pont, and the like) are generally used as the polyelectrolyte membrane 51. A gas diffusion layer 53 is formed on an outer surface of the catalyst layer 52, the gas diffusion layer 53 being a carbon paper having both gas permeability and electron conductivity, for example, such as a carbon paper having been subjected to a water-repellent treatment. Combination of the catalyst layer 52 and the gas diffusion layer 53 is called an electrode 54.
Next, to prevent leakage of a fuel gas and an oxidant gas supplied, to the outside and to prevent the above two kinds of gases from being mixed, a gas sealant or a gasket is provided on the circumference of the electrode sandwiching the polyelectrolyte membrane. This sealant or gasket is previously fabricated together with the electrode and the polyelectrolyte membrane as a unit, and the assembly combining all of those is called MEA (electrolyte membrane electrode assembly) 55.
As shown in FIG. 6, a conductive separator plate 56 for mechanically fixing MEA 55 is provided on the outside of MEA 55. A gas channel 57 for supplying a reaction gas to the face of the electrode and carrying away a produced gas or an excess gas is formed on a contact part of the separator plate 56 and MEA 55. The gas channel can be provided separately from the separator plate, but a system of forming a gas channel by providing grooves on the surface of the separator plate is generally utilized. Thus, by fixing MEA 55 with a pair of the separator plates 56, supplying a fuel gas to one side of the gas channel 57, and supplying an oxidant gas to the other side of the gas channel 57, an electromotive force of about 0.7 to 0.8V can be generated with one unit cell when applying current of a practical current density of from several tens to several hundreds of mA/cm2. A cooling water channel 58 is provided on the face of the separator 56 not facing MEA to circulate cooling water.
What is obtained by fixing MEA 55 with a pair of the separator plates 56 is called a unit cell. However, in general, when a fuel cell is used as a power source, several to several hundreds of volts is required. Therefore, practically, the required number of unit cells are connected in series.
To supply a gas to the gas channel, a piping jig is required, which branches a pipe supplying a gas into the number corresponding to the number of the separator plates used, and directly connects the branched heads to the grooves on the separator. This jig is called a manifold, and in particular, a manifold of the type directly connecting from a pipe for supplying a gas as above is called an external manifold. Further, a manifold having a simpler structure is called an internal manifold. The internal manifold is so designed that through-holes are provided on the separator plate having gas channels formed thereon, an inlet and outlet of the gas channel is extended up to this hole, and a gas is directly supplied to the gas channel through this hole.
FIG. 7 is a schematic cross-sectional view of a fuel cell to which load 512 has been connected, and explains with respect to a gas diffusion layer 501 and a catalyst layer 502, constituting an anode 509 and a cathode 510 of a fuel cell by sandwiching a polyelectrolyte membrane 511 from the both sides. The gas diffusion layer 501 mainly has the following three functions. The first function is a function that diffuses a reaction gas in order to uniformly supply the reaction gas such as a fuel gas or an oxidant gas to a catalyst 503 in a catalyst layer 502 from a gas channel positioned on further outer surface of the gas diffusion layer 501. The second function is a function that quickly discharges water formed by reaction in the catalyst layer 502 into the gas channel. The third function is a function that conducts electrons necessary for reaction or electrons to be generated. That is, high reaction gas penetrability, water discharging properties and electron conductivity are required for the gas diffusion layer 501.
As a conventional general technique, to give gas permeability, a porous structure is given to the gas diffusion layer 501 by using a conductive porous substrate such as a carbon fine powder having developed structure constitution, pore-forming agents, a carbon paper and a carbon cloth, as the gas diffusion layer 501. Further, to give water discharging properties, for example, a water-repellent polymer represented by a fluororesin is dispersed in the gas diffusion layer 501 or the like. Further, to give electron conductivity, the gas diffusion layer 501 is constituted with an electron conductive material such as a carbon fiber 505, a metal fiber and a carbon fine powder.
Next, the catalyst layer 502 mainly has the following four functions. The first function is a function that supplies a reaction gas such as a fuel gas or an oxidant gas, supplied from the gas diffusion layer 501 to a reaction site of the catalyst layer 502. The second function is a function that conducts hydrogen ions necessary for reaction on a catalyst 503 or hydrogen ions to be generated. The third function is a function that conducts electrons necessary for reaction or electrons to be generated. The fourth function is high catalyst performance for speeding an electrode reaction and its wide reaction area. That is, high reaction gas permeability, hydrogen ion conductivity, electron conductivity, and catalyst performance are required for the catalyst layer 502.
As a conventional general technique, to give gas permeability, constituting a gas channel 507 is performed by using a catalyst carrier 504 of carbon fine powder having developed structure constitution or pore-forming agents to give a porous structure to the catalyst layer 502. Further, to give hydrogen ion penetrability, a polyelectrolyte is dispersed in the vicinity of the catalyst 503 in the catalyst layer 502, and hydrogen ion network 508 is formed.
Further, to give electron conductivity, an electron conductive material such as carbon fine powder and carbon fibers is used, as a catalyst carrier 504, thereby constituting an electron channel 506. Further, to improve catalyst performance, a metal catalyst 503 having high reaction activity represented by platinum is carried on carbon fine powder as very fine particles having a particle diameter of several nm, and the catalyst body obtained is highly dispersed in the catalyst layer 502.